1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator for surface mounting (hereinafter called a “temperature-compensated oscillator”) and, in particular, to a surface-mount type oscillator that provides temperature compensation reliably at the startup thereof.
Since temperature-compensated oscillators for surface mounting are compact, lightweight, and have a high degree of frequency stability with respect to temperature changes, they are used as built-in frequency sources in portable devices such as mobile phones, particularly in changing temperature environments. One type is a temperature-compensated oscillator in which a temperature-compensating device and an oscillating circuit are integrated into an IC chip which is hermetically sealed in together with a crystal piece.
2. Description of Related Art
A temperature-compensated oscillator of the prior art is shown illustratively in FIG. 6, where FIG. 6A is a section through the temperature-compensated oscillator for surface mounting, FIG. 6B is a schematic diagram of circuit blocks used therein, FIG. 6C is an enlarged plan view as seen from below of an IC chip used therein, and FIG. 6D is a plan view of a crystal piece used therein.
This temperature-compensated oscillator has a configuration such that an IC chip 102 and a crystal piece 103 are housed in a main container 101 that is concave in section, having an inner wall step portion, and are covered by a metal cover 104. The main container 101 is formed of stacked ceramic layers 101a, 101b, and 101c, has mounting terminals 105 on the outer sides of the base surface thereof, and also writing terminals (not shown in the figure) for writing temperature compensation data on an outer side surface thereof. Circuit terminals (not shown in the figure) are provided on an inner base surface 101d of the main container 101 and also crystal-holding terminals (not shown in the figure) are provided on an inner wall step portion thereof.
The IC chip 102 has IC terminals 115 on one main surface thereof that is a circuit function surface, and is configured to have an oscillating circuit 106 and a temperature-compensating device 107 therein, excluding a crystal oscillator (the crystal piece 103). The IC terminals 115 include at least a pair of crystal terminals, and power source, output, and ground terminal, and also writing terminals or the like for writing temperature compensation data. The IC terminals 115 are formed on the sides of two edges that include at least diagonally opposite corner portions of one main surface of the IC chip 102, which is the circuit function surface thereof (see FIG. 6C).
The one main surface of the IC chip 102 is then placed face-down on the inner base surface 101d of the main container 101, and the IC terminals 115 are connected to the circuit terminals on the inner base surface 101d of the main container 101 by means such as ultrasonic thermal crimping using bumps 108 made of gold. Solder is applied to at least the surfaces of the bumps 108 and the affixing is done by reflow. The IC terminals 115 are connected to the corresponding mounting terminals 105, crystal-holding terminals, and writing terminals of the main container 101.
In this case, resin (as “underfill”) that protects the circuit function surface is not applied between the one main surface of the IC chip 102 and the inner base surface 101d of the main container 101. This is because the IC chip 102 will be sealed into the main container 101 together with the crystal piece 103, and thus will be shielded and protected from the exterior, so there is no need for underfill.
The oscillating circuit 106 shown in FIG. 6B is formed from an inverter amplification element that is a CMOS element and a feedback circuit (not shown in the figure), by way of example. The resonance circuit is formed of the crystal oscillator (the crystal piece 103) and a dividing capacitor that is incorporated into the IC chip 102. The temperature-compensating device 107 has a temperature sensor is formed of a resistor or the like that detects the ambient temperature, to generate a compensation voltage Vc in response to that ambient temperature. The compensation voltage Vc is generated in accordance with temperature compensation data from the writing terminals, based on previously measured temperature characteristics.
As shown in FIG. 6D, the crystal piece 103 has a pair of symmetrical excitation electrodes 109 on the two main surfaces thereof, each with an output electrode 110 extending to either side of one end portion thereof. The two sides of the one edge portion to which the output electrodes 110 extend are affixed by an electrically conductive adhesive 11 to the crystal-holding terminals on the inner wall step portion of the main container 101, as shown in FIG. 6A. These crystal-holding terminals are connected to the crystal terminals of the IC chip 102 and form a resonance circuit together with a dividing capacitor. The metal cover 104 is affixed by means such as seam welding to a metal ring 112 provided on the aperture edge surface of the main container 101.
In the thus-configured prior-art temperature-compensated oscillator, the compensation voltage Vc that is based on changes in the resistance detected by the temperature sensor of the temperature-compensating device 107 (i.e., the ambient temperature) is applied to a variable-voltage capacitor element 113 that has been inserted within the oscillating circuit 106 (oscillation loop). Since the load capacitance varies as seen from the crystal oscillator, the frequency-temperature characteristic which is particularly dependent on the crystal oscillator (the crystal piece 103) and which forms a curve such as a cubic curve is flattened, and the frequency stability with respect to temperature can be increased. (See Japanese Patent Laid-Open Publication No. 2003-101348 (such as FIG. 1))